1. Field of the Invention
The present invention relates to thermally insulated transport systems, insulated containers and insulation structures. In particular, the present invention relates to thermally insulated structures where the thermal insulation works in a reduced pressure environment and certain segments of the insulation system which display reduced performance over time can be regenerated without replacement of the thermal insulation system.
2. Background of the Art
Thermal insulation is widely used throughout all aspects of technology and sciences. Every structure and device from housing to superconductors involves consideration of the need for avoiding undesirable heat transfer within the system. The fundamental physics of thermal insulation can be usually resolved in the single consideration that thermal transfer across any volume will be minimized if the mass within that volume is minimized. Heat transfer by both conduction and convection are eliminated in the absence of mass surrounding the mass having heat energy. Only radiant energy can pass over the volume, and that can be reduced by the proper arrangement of reflectors and black body absorbers.
Reduced mass within the insulating volume is used both with cold storage systems and high temperature systems. The structures for reducing heat transfer to or from a volume or area generally comprise a central container with walls (including pipes, tubing, refrigeration elements, transient storage containers such as boilers, condensers and furnaces) where the reduced transmission of heat is important. Around the walls is an insulating zone. The primary objective of the insulating zone is to provide the minimum amount of mass, and the minimum amount of thermally conductive mass, between the outside walls of the central container and an outer shell, which is usually the visible external walls of the device or system. The volume between the outer surfaces of the central container and the outside walls is the section of the device or system containing insulation. The insulation may take many forms, such as a vacuum (with a minimum number of thermally insulating contact or support points separating the outside surface of the central container and the inside surface of the outside walls), a highly porous material, such as a foam (e.g., polyurethane, polystyrene, ureaformaldehyde, etc.), reticulated structures (such as blown microfibers, foams with collapsed cell walls, etc.), fibrous material (synthetic non-woven materials, fiberglass, ceramic fibers, and natural materials such as asbestos), and the like. The structure and composition of each of these types of insulation still works on the principle that the lowest volume of mass (especially gases which can readily convey energy through mass transfer) and the use of the most thermally insulating solid materials will provide the best insulation.
In systems which rely most strongly upon the presence of reduced pressure or a vacuum to provide insulation between the central container and the outside walls, it is important to keep the specific level of reduced pressure at a minimum and to keep that pressure constant. This is particularly true in cryogenic systems where temperatures below -50, -75, -100.degree. C., or lower are used. Even though a vacuum may be originally presented within the system, there can be extremely small leaks, vapor pressure generated by volatiles or ingredients within the insulation zone (e.g., plasticizers on polymers and adhesives, the natural vapor pressure of atomic or molecular materials, unreacted ingredients in coatings, degradation products from materials, etc.), and the like. The addition of these types of materials to the vacuum zone or insulation zone are particularly annoying because they change over time. In systems where temperature control is critical (as in chemical reaction systems, superconductive electric transmission systems, laser systems, cryogenic storage, and the like), fluctuations in the insulating properties can alter critical temperature requirements for the system, and these changes vary irregularly over time. Because they change irregularly over time, adjustments to the system must usually be effected periodically, with high labor utilization, and these corrections and adjustments can be inexact.
One way of addressing this type of variation in the vacuum over time has been to place a packet of absorbent material (e.g., referred to in the art as a "getter"). Getters are materials which usually chemically react with expected molecular contaminants within the vacuum area and thus remove them from the air. Getters typically react with materials by activation upon heating, as compared to absorbents for gases which work more efficiently when the temperature drops. With absorbents, the lower the temperature, the greater the weight of gas which can be absorbed. These getters, in some cases, happen to be materials from which the reacted chemicals can be driven by heating the getter outside of the container to reverse the chemical reaction which bonds the contaminants to the getter. Where the system is completely closed, these getters will eventually fill up, and replacement of the packets of getters is time consuming and somewhat inefficient, since after opening the system, the packet of getters is inefficient in cleansing out the entire vacuum zone.